1. Field of the Invention
The present invention relates to a walking beam rod pumping assembly used in oil fields. More specifically, the invention relates to a double stage balanced rod pumping assembly which can have its balance adjusted in a non-stopped status.
2. Related Art
Conventional walking beam pumping assemblies are commonly used in oil fields. The varieties of the conventional walking beam pumping assemblies include: out of phase crank balanced walking beam pumping assemblies; front driven walking beam pumping assemblies; and pneumatically balanced walking beam pumping assemblies. The feature common to all conventional walking beam pumping assemblies is that the movement changing device is simple in structure, i.e., it is a four-hinge crank-rocker mechanism including a main crank, a connecting rod, a rocker (walking beam or loose pulley), and a stand (formed integrally with a base). Thus, conventional pumping assemblies can be referred to as walking beam (loose pulley) pumping assemblies with four-hinge crank-rocker mechanisms. Since the movement changing device of conventional pumping assemblies is very simple in structure, this kind of pumping assembly has the advantages of reliable structure, convenience in production, simple maintenance and durability and adaptability to various bad conditions of the oil field.
The disadvantages of a conventional walking beam pumping assembly with a four-hinge crank-rocker mechanism are that the fluctuations of torque are large, with large negative torques and large expenditures of power, and the conventional devices require heavy steel structural components.
Chinese Patent Application No. 88107522 (granted to the same applicant on Jun. 24, 1993) disclosed a double stage balance rod pumping assembly, which can eliminate the large negative torques of conventional pumping assemblies, lower the fluctuational range of the torques, save electrical energy expended, and reduce the total weight of the assembly greatly. Therefore, the pumping assembly disclosed in Chinese Patent Application No. 88107522 has overcome many of the disadvantages of the conventional walking beam pumping assemblies having a four-hinge crank-rocker mechanism. However, this kind of pumping assembly can't have its balance adjusted while in a non-stopped status and thus is limited in its ability to benefit from the effect of double stage balancing. The primary and secondary balance torques arc periodic functions of the rotation angles of the cranks having the periods of 2.pi. and .pi. respectively.